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std::equal_range

From cppreference.com
< cpp‎ | algorithm
Revision as of 18:58, 18 August 2013 by Ggao (Talk | contribs)

 
 
 
Defined in header <algorithm>
template< class ForwardIt, class T >

std::pair<ForwardIt,ForwardIt>
    equal_range( ForwardIt first, ForwardIt last,

                 const T& value );
(1)
template< class ForwardIt, class T, class Compare >

std::pair<ForwardIt,ForwardIt>
    equal_range( ForwardIt first, ForwardIt last,

                 const T& value, Compare comp );
(2)

Returns a range containing all elements equal to value in the sorted range [first, last). The range is defined by two iterators, one pointing to the first element that is not less than value and another pointing to the first element greater than value. The first iterator may be alternatively obtained with std::lower_bound(), the second - with std::upper_bound().

The first version uses operator< to compare the elements, the second version uses the given comparison function comp.

Contents

Parameters

first, last - the range of elements to examine
value - value to compare the elements to
comp - comparison function which returns ​true if the first argument is less than the second.

The signature of the comparison function should be equivalent to the following:

bool cmp(const Type1 &a, const Type2 &b);

The signature does not need to have const &, but the function must not modify the objects passed to it.
cmp will be called as both cmp(value, *iterator) and cmp(*iterator, value).


Type requirements
-
ForwardIt must meet the requirements of ForwardIterator.

Return value

std::pair containing a pair of iterators defining the wanted range, the first pointing to the first element that is not less than value and the second pointing to the first element greater than value.

If there are no elements not less than value, last is returned as the first element. Similarly if there are no elements greater than value, last is returned as the second element

Complexity

Logarithmic in the distance between first and last

Possible implementation

First version
template<class ForwardIt, class T>
std::pair<ForwardIt,ForwardIt> 
    equal_range(ForwardIt first, ForwardIt last,
                const T& value)
{
    return std::make_pair(std::lower_bound(first, last, value),
                          std::upper_bound(first, last, value));
}
Second version
template<class ForwardIt, class T, class Compare>
std::pair<ForwardIt,ForwardIt> 
    equal_range(ForwardIt first, ForwardIt last,
                const T& value, Compare comp);
{
    return std::make_pair(std::lower_bound(first, last, value, comp),
                          std::upper_bound(first, last, value, comp));
}

Example

#include <algorithm>
#include <vector>
#include <iostream>
 
struct S
{
    int number;
    char name;
 
    S ( int number, char name  )
        : number ( number ), name ( name )
    {}
 
    // only the number is relevant with this comparison
    bool operator< ( const S& s ) const
    {
        return number < s.number;
    }
};
 
 
int main()
{
    std::vector<S> vec = { {1,'A'}, {2,'B'}, {2,'C'}, {2,'D'}, {3,'F'}, {4,'G'} };
 
    S value ( 2, '?' );
 
    auto p = std::equal_range(vec.begin(),vec.end(),value);
 
    for ( auto i = p.first; i != p.second; ++i )
        std::cout << i->name << ' ';
}

Output:

B C D

Example With Comparator

#include <algorithm>
#include <vector>
#include <iostream>
 
struct S
{
    int number;
    char name;
 
    S ( int number, char name  )
        : number ( number ), name ( name )
    {}
 
    // only the number is relevant with this comparison
    bool operator< ( const S& s ) const
    {
        return number < s.number;
    }
};
 
struct Comp
{
    bool operator() ( const S& s, int i )
    {
        return s.number < i;
    }
 
    bool operator() ( int i, const S& s )
    {
        return i < s.number;
    }
};
 
int main()
{
    std::vector<S> vec = { {1,'A'}, {2,'B'}, {2,'C'}, {2,'D'}, {3,'F'}, {4,'G'} };
 
    auto p = std::equal_range(vec.begin(),vec.end(),2,Comp());
 
    for ( auto i = p.first; i != p.second; ++i )
        std::cout << i->name << ' ';
}

Output:

B C D

See also

returns an iterator to the first element not less than the given value
(function template) [edit]
returns an iterator to the first element greater than a certain value
(function template) [edit]
determines if an element exists in a certain range
(function template) [edit]